The present invention relates to the use of palladium (Pd) in ferroelectric memory devices and in particular, ferroelectric capacitors.
Ferroelectric capacitors are of interest today because of their use in ferroelectric memories and thin film discrete capacitors. See U.S. Pat. Nos. 4,873,664; 4,809,225; 4,853,893; 4,918,654; 4,910,708; 4,914,627; 4,893,272; 4,888,733, all issued to Ramtron Corporation of Colorado Springs, Colo., USA.
Platinum (Pt) electrodes are often used in the fabrication of ferroelectric capacitors. Unfortunately, currently a number of problems exist with the use of these Pt electrodes in ferroelectric capacitors, not the least of which is the high cost of platinum.
Ferroelectric capacitors are fabricated using an oxide known as "PZT" which refers generally to a mixture or solution comprised of oxides of lead, zirconium and titanium, forming a perovskite structure. The PZT is sandwiched between electrodes typically comprised of noble metals, termed lower and upper electrodes. The lower electrode may be deposited directly on silicon dioxide or some other suitable dielectric. When using Pt (or another noble metal) in the fabrication of a ferroelectric capacitor, the adhesion at the interface between the Pt or other noble metal and the dielectric is poor. If there is poor adhesion at this interface, delamination can potentially occur between the electrode and the dielectric. In addition, the adhesion of the electrode/dielectric/electrode to the substrate is also poor. The substrate typically comprises silicon covered with a layer of silicon dioxide which has a thickness in the range between 5,000 .ANG. (500 nm) and 8,000 .ANG. (800 nm).
Presently, one approach for solving this adhesion problem is to use titanium (Ti) or chromium (Cr) under the Pt as a "glue" layer. However, both Ti and Cr can readily diffuse through the Pt. If the Ti or Cr does diffuse through the Pt and is able to reach the PZT interface, the Ti or Cr will oxidize and will result in the degradation of the electrical properties of the capacitor.
One possible solution to the adhesion problem between the electrodes and the dielectric is disclosed in Ramtron's co-pending U.S. patent application Ser. No. 07/714,682, entitled "Conducting Electrode Layers For Ferroelectric Capacitors In Integrated Circuits And Method" which is incorporated herein by reference.
Accordingly, the main object of the present invention is to create a ferroelectric capacitor structure that overcomes the deficiencies of the prior devices.